This invention relates to solid particle dispersions and their use in the preparation of aqueous coatable laser thermal media.
In recent years thermal transfer systems have been developed to obtain prints from pictures which have been generated electronically from color video cameras. According to one way of obtaining such prints, an electronic picture is first subjected to color separation by color filters. The respective color-separated images are then converted into electrical signals. These signals are then operated on to produce cyan, magenta and yellow electrical signals. These signals are then transmitted to a thermal printer. To obtain the print, a cyan, magenta or yellow dye-donor element is placed face to face with a dye-receiving element. The two are then inserted between a thermal printing head and a platen roller. A line-type thermal printing head is used to apply heat from the back of the dye-donor sheet. The thermal printing head has many heating elements and is heated up sequentially in response to the cyan, magenta and yellow signals. The process is then repeated for the other two colors. A color hard copy is thus obtained which corresponds to the original picture viewed on a screen. Further details of this process and an apparatus for carrying it out are contained in U.S. Pat. No 4,621,271.
Another way to thermally obtain a print using the electronic signals described above is to use a laser instead of a thermal printing head. In such a system the donor sheet includes a material which strongly absorbs at the wavelength of the laser. When the donor is irradiated, this absorbing material converts light energy to thermal energy and transfers the heat to the dye in the immediate vicinity, thereby heating the dye to its vaporization temperature to transfer to the receiver. The absorbing material may be present in a layer beneath the dye and/or it may be admixed with the dye. The laser beam is modulated by electronic signals which are representative of the shape and color of the original image, so that each dye is heated to cause volatilisation only in those areas in which its presence is required on the receiver to reconstruct the color of the original object. Further details of this process are found in GB 2083726A.
In one ablative mode of imaging by the action of a laser beam, an element with a dye layer composition comprising an image dye, an infrared-absorbing material and a binder coated onto a substrate is imaged from the dye side. The energy provided by the laser drives off substantially all of the image dye and binder at the spot where the laser beam hits the element. In ablative imaging, the laser radiation causes rapid local changes in the imaging layer thereby causing the material to be ejected from the layer. Ablation imaging is distinguishable from other material transfer techniques in that some sort of chemical change (eg bond breaking) rather than a completely physical change (eg melting, evaporation, sublimation) causes an almost complete transfer of the image dye rather than a partial transfer. The transmission Dmin density value serves as a measure of the completeness of image dye removal by the laser.
U.S. Pat. No. 5,468,591 relates to a direct image recording film which has a barrier layer between the support and the image recording layer to prevent downward migration of dyes during the ablation process. The barrier layer itself is partially or completely ablated during imaging, resulting unacceptably low Dmin values. A typical barrier layer disclosed includes an infrared (IR)-absorbing dye and a binder copolymer of 80:20 wt % poly (methyl cyanoacrylate-coethyl cyanoacrylate). However, there is a problem with the above prior art barrier layer in that it is applied by coating from organic solvents such as acetonitrile or acetone. The use organic solvents in production coating is undesirable because of fire and health risks. Also the solvent evaporating from the coating must be captured during manufacturing to prevent pollution, which is an expensive process. A solution to this problem is disclosed in U.S. Pat. No. 5,712,079 which describes the preparation of a barrier layer using an aqueous latex dispersion of a polycyanoacrylate polymer. However this requires that on preparation of the polymer, the polymer solution is deliberately diluted to 20% solids with ethyl acetate and methanol to enable the preparation of the latex loaded dispersions. This extra volatile organic solvent then needs to be removed by evaporation which means an extra step is required.
U.S. Pat. No. 6,068,967 describes the use of polymers containing di-carboxylic acid groups incorporated as preformed metal salts in the preparation of solid particle dispersions of filter dyes where they confer small particle size and improved wash out for silver halide photographic material.
U.S. Pat. No. 5,657,931 describes dispersants to provide stable solid particle dispersions. The polymer surfactants described are based on alkylene oxide groups and are not entirely suitable for the dual purpose role fulfilled by the polymers used in the present invention.
Problem to be Solved by the Invention
For environmental reasons it is desirable to reduce further the use of volatile organic solvents and the present invention provides a solution to this problem by the provision of a method for the production recording elements using aqueous solid particle dispersions.
According to the present invention there is provided an aqueous solid particle dispersion suitable for application to a substrate to form a coating said dispersion comprising: a polymer, an infrared-absorbing material and a surfactant to stabilize the dispersion and wherein the polymer is a copolymer of (i) a monomer that will confer thermally degradability and (ii) a monomer that will confer water dispersibility.
The aqueous dispersions have the advantages of simplicity of preparation being made by an essentially one pot method and their reduced amounts of volatile organic components.